1. Field of the Invention
The present invention relates to an image forming apparatus and electrophotographic method for forming an electrostatic latent image on an optical body and developing the electrostatic latent image with toner. More specifically, the present invention relates to an image forming apparatus and back exposure method for exposing the back surface of the photosensitive body to light.
2. Related Background Art
As means for obtaining a hard copy, such as a copying machine, a computer, and so on, image forming apparatus utilizing the electrophotographic method have been widely used. A typical image forming apparatus has a photosensitive body and devices for image formation arranged around the photosensitive body. More specifically, around the photosensitive body, there are provided a charger device, an exposure device, a developer device, a transfer device, a cleaning device, and so on. This-image forming apparatus and electrophotographic method carriers out an image forming process comprising steps of charging the photosensitive body with electricity, exposing the photosensitive body to light in order to form an electrostatic latent image on the photosensitive body, developing the resultant electrostatic latent image by applying toner to it to obtain a toner image, transferring the developed toner image onto a transfer medium, and subsequently fixing the transferred toner image on the transfer medium to finally obtain a print image.
Compared with print images obtained from other means for obtaining hard copies, such as those of thermal transfer type, of ink jet type, of impact printing type, or the like, the print image obtained as described above has higher resolution and stronger contrast, that is, as a whole, high quality.
However, as described before, the image forming process by the electrophotographic method requires many devices. So, the apparatus therefor tends to be of a large size and complicated. It is not easy to miniaturize and simplify the apparatus.
In order to solve this problem, some methods have been proposed in which, while using the same electrophotographic method, the apparatus carries out all the processes such as electrification, exposure development, and so on substantially at the same time and at the same position (such combined processes will be referred to "simplified process"). Among such methods, typical ones are disclosed, for example, in Japanese Laid-Open Patent Appln. Nos. 58-153957, 62-209470 and so on. In general, in these methods, either conductive toner or conductive carriers, and insulating toner are used, and the image forming process comprises steps of (1) cleaning the residual toner which was not transferred during the previous image forming process; (2) contact electrification; (3) image exposure from the back surface of the photosensitive body; and (4) contact development. The series of steps are performed in a developing nip between the photosensitive body and a magnetic brush roller which corresponds to an exposure position on the back surface of the photosensitive body and which is in contact with the outer surface of the photosensitive body.
More specifically, as shown in FIG. 2, a magnetic brush provided upstream in the developing nip N between a developer sleeve 22 and a photosensitive body 1 scrapes the residual toner which was not transferred (hereinafter referred to as "transfer residual toner") to clean the photosensitive body 1. As the toner employed here is magnetic toner T and a fixed magnet 23 is arranged inside the developer sleeve 22, magnetic force can improve the cleaning effect.
Then, the surface of the photosensitive body 1 is brushed by a conductive magnetic brush (of conductive toner or conductive carriers) to apply electric charge to the surface of the photosensitive body 1. As the electrification is carried out by trapping electric charge in impurity levels on the surface of the photosensitive body 1, charger member(s) having very small resistance and a long period of electrification are required to carry out electrification sufficiently. Therefore, material which sufficiently holds electricity near its surface is needed. As such a material, amorphous silicon (hereinafter referred to as "a-Si"), selenium, and so on are preferably used.
The above-mentioned cleaning operation and electrification are performed at the same time in a cleaning-electrification region Nc, which is in the developing nip N and upstream with respect to a back surface exposure position A (described later). Incidentally, the potential of the charged photosensitive body 1 brushed with the magnetic brush is substantially equal to the applied voltage or less.
Next, the back surface of the photosensitive body 1 is exposed to light. A light source (exposure means) 3 having an LED array 31 illuminate the predetermined position (back surface exposure position) in the developing nip N formed by developer between the developer sleeve 22 and the photosensitive body 1. Thus, a latent image is formed on the exposed photosensitive body 1. The latent image is developed in a development region N.sub.D, which is downstream with respect to the back surface exposure position A, in the developing nip N. When conductive toner is used, the electric charge electrostatically induced by the latent image formed on the photosensitive body 1 is applied via a triboelectric brush to the toner at the tip of the triboelectric brush. The latent image is developed with toner separated from the triboelectric brush by Coulomb force acting between the electric charge and the electric charge of the latent image.
Otherwise, when two-component developer consisting of magnetic conductive carriers C and insulating toner T is used in the same apparatus, the triboelectric brush of the conductive carriers serves, as neighboring electrodes. Accordingly, sufficient electrical field for development can be obtained even if the voltage applied between the photosensitive body 1 and the developer sleeve 22 is small. Thus, development with insulating toner can be carried out by applying low voltage.
Since it is difficult to transfer the toner image formed on the photosensitive body 1 onto the transfer medium P in the electric field obtained when conductive toner is employed, development with two-component developer including insulating toner is generally preferred.
OPC photosensitive bodies of the functionally separated type, which are recently most widely used as photosensitive bodies, are hard to apply with electric charge. So, they have not been generally used in the image forming apparatus as described above. But, it has been proved that by forming an electric charge supply layer on the surface, the electric charge supply characteristic of the OPC photosensitive bodies can be improved to realize sufficient electrification. There is another problem concerning the simplified process according to the prior art; fogging in non-image portions. The simplified process comprising steps as described above can not realize "reverse contrast" which is generally employed in the electrophotographic process. Accordingly, fogging easily occurs in non-image portions.
For example, an image was formed with the apparatus shown in FIG. 2: volume resistivity of the conductive particles employed, 10.sup.3 .OMEGA..multidot.cm; an a-Si photosensitive body coated with a silicon calcium carbide; voltage applied to the developer sleeve, +60 V. The photosensitive body was charged with voltage V.sub.D of +55 V, while exposed portions thereof were charged with voltage V.sub.L of +20 V. The photosensitive body charged with such voltages was subjected to development in the development region N.sub.D downstream in the developing nip N. That is, positive toner was reversed to develop the latent image with a design of potentials shown in FIG. 3: non-image forming portions, V.sub.D =+55 V; image forming portions (exposed portions), V.sub.L =+20 V; and developing potential applied V.sub.DC =+60 V.
As is clearly seen from the potential design, a development contrast of 50 V was obtained, while no reverse contrast exsisted. The potential V.sub.D of the non-image forming portions was 5 V lower than the developing potential V.sub.DC, wherein the non-image portions might be developed. Actually, the magnet inside the developer sleeve inhibits the magnetic toner from developing the non-image forming portions. Nevertheless, fogging may easily occur in the above-mentioned simplified process.
As described above, though the simplified process can be carried out with a simple apparatus, images of good quality can not be reliably obtained because of fogging.